1. Field of the Invention
This invention relates to servo control for the optical pickup of optical disc drives, and more particularly, to an auto gain adjustment apparatus and method for use in the focusing/tracking control of the optical pickup of an optical disc drive.
2. Description of Related Art
The optical disc is a mass storage medium that can hold up to several megabits or even gigabits of binary data in a single piece of plastic disc, which is several times larger than a conventional magnetic disk can hold. Due to its high capacity, the optical disc is now widely used in the computer arena as the major data storage medium.
An optical disc drive is composed of a large number of constituent parts including motors and optical and electronic components. In operation, the characteristics of these constituent parts can change in time and are affected by various environmental conditions. The changes in the characteristics can then affect the gain of the servo control loop for focusing/tracking control of the optical pickup of the optical disc drive. Moreover, the gain can be further affected by changes in the optical properties of the photo sensors and the reflectivity of the optical disc being used. Therefore, the gain of the servo control loop should be always adaptively adjusted to compensate for any changes in the system characteristics before being put into actual operation so as to allow the optical disc drive to operate without errors in focusing and tracking.
The adjustment of the gain of the servo control loop in the optical disc drive is conventionally done in either of the following two ways: either through manual adjustment, or through the application of an externally generated oscillating test signal, such as a sinusoidal signal or a square pulse train, to the system. It is apparent that the manual adjustment would be highly cost-ineffective since it would involve great amounts of labor works. The latter method is more advantageous in terms of efficiency, but is still unsatisfactory in that the application of the externally generated oscillating test signals to the system is still quite inconvenient and time-consuming. This drawback will be illustratively depicted in the following with reference to FIG. 1.
FIG. 1 is a schematic block diagram of a conventional gain adjustment apparatus for adapatively adjusting the gain of the servo control loop for focusing/tracking control of the optical pickup of an optical disc drive. As shown, the gain adjustment apparatus includes a bandpass filter (BPF) 10, a adder 101, a sinusoidal signal generator 11, a phase comparator 12, an averaging circuit 13, a judgment circuit 14, and a gain adjusting unit 15 with a variable gain K under control by the judgment circuit 14.
The input signal to this gain adjustment apparatus is denoted by FE/TE (focusing/tracking error), which is outputted from the optical pickup (not shown) of the optical disc drive during focusing/tracking operation. The FE/TE signal is first processed by the BPF 10 which can filter out all the undesired frequency components beyond the designated bandwidth of the FE/TE signal. The FE/TE signal is also added with an output of the sinusoidal signal generator 11 by the adder 101, and then a mixed FE/TE signal is generated. The phase comparator 12 then compares the phase of the output of the BPF 10 with the phase of the output of the sinusoidal signal generator 11. The output of the phase comparator 12 is then averaged by the averaging circuit 13 which sends the averaged value to the judgment circuit 14. In response, the judgment circuit 14 generates a corresponding gain control signal based on the output of the averaging circuit 13. The gain control signal is then sent to the gain adjusting unit 15 to control the gain K thereof to a corresponding level in response to the gain control signal so as to cause the gain adjusting unit 15 to amplify the mixed FE/TE signal by the gain K.
One drawback to the foregoing gain adjustment apparatus, however, is that the use of the sinusoidal signal generator 11 is still considered inconvenient, in that externally generated test signals, such as a sinusoidal signal or square pulse train, needs to be applied to measure the system characteristics.